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Long rise time with LVTH bus hold

Wade Vonbergen
Guru 62930 points

I am trying to help validate a customer design.

They are using an LVTH244 as a buffer in an I2C circuit.  The source driver is open drain, and thus the circuit has a required pullup that is not recommended for bus hold circuits.  The pullup is sourcing ~6ma, so this should not be a concern with conflicting with the bus hold current.


The real question comes in with the rise time.  The circuit has a slew rate of about 72ns/V.   This is significantly greater than the ROC value of 10ns/V max.

However, there is not an absolute max slew rate defined.

Does this pose a reliability risk with slow rise time?

Does this pose a risk of oscillation on output, or does any hysteresis introduced from the bus hold feedback prevent this?

Thanks,

Wade

  • 0
    TI__Guru 62160 points
    Hi Wade,

    would like to know why was the bushold device chosen if there is not going to be tristate condition? I would suggest to include a Schmitt trigger input to avoid the slow rising edge signals which could cause issues in terms of oscillations on the output, false triggering , higher current consumption.
    Having pullups in front of Schmitt triggers would be fine to not cause issues on outputs... other devices like AUC244 or AHC244 could also be considered which have better input transition rate compared to LVC device family.
  • 0 in reply to ShreyasRao
    TI__Guru 62930 points

    Thanks Shreyas.

    I did not mention this part was chosen based on it being a space grade device.  This device was chosen as it has ioff capability.  There is not an equivalent -sp device without bushold and ioff.

    Any information  on if there are oscillation or reliability issues?

    Thanks,

    Wade

  • 0 in reply to Wade Vonbergen
    TI__Guru 62160 points
    Hi Wade ,

    I am not qualified to speak for the high reliability device , however , I would say that since the ROC is violated , TI wouldn't be able to guarantee the performance without issues. However would also like to add that TI devices are robust enough to handle this shift from the ROC.
  • 0 in reply to ShreyasRao
    TI__Guru 62930 points

    Thanks Shreyas.

    That was my interpretation as well.  I just wanted to double check with you in case this has been addressed prior.


    Regards,

    Wade